The present invention relates to charge comparators, especially those which may be used with Charge Coupled Device (CCD) pipeline-based structures.
Circuits for performing signal processing functions are now common in numerous consumer devices such as digital cameras, cellular telephones, wireless data network equipment, audio devices such as MP3 players, video equipment such as Digital Video Disc (DVD) players, High Definition Digital Television (HDTV) equipment and numerous other products. It is well known that Charge Coupled Devices (CCDs) circuits may be used to implement many of the signal processing functions required in such products.
One circuit, called a Charge to Digital (QDC) converter, is used to convert an input analog voltage to digital data. One way to implement a QDC is as a successive approximation type converter that includes a number of charge storage stages arranged as a serial pipeline register. The input signal voltage is typically represented as a pair of complimentary charges which are processed in positive (plus) and negative (minus) signal paths. In this type of converter, the complimentary input charges pass from stage to stage down respective pipelines dedicated to processing the plus and minus signals.
One critical component of a QDC is the reference charge comparator used at each stage. The reference charge comparator compares an input charge amount to a reference charge amount, and then optionally adds a reference charge amount to each charge as it travels through the stage.
Accurate comparison of the charge input to each stage is therefore an extremely important part of assuring overall QDC accuracy. Unfortunately, Direct Current (DC) and/or Analog Current (AC) offset input errors can be introduced into these charge comparator circuits quite easily.
In a differential QDC, if a different input offset is introduced in the plus path than in the minus path, this can also result in erroneous results. Even a slight offset in the amount of a reference charge comparison result can thus have a ripple effect as the charges pass down multiple pipeline stages.